Conventional methods of preparing thermoplastic polymeric film include casting a sheet of molten polymer through a slit die onto a cooled quenching surface to stabilize and solidify the film. It is desirable to bring the freshly extruded web into close and rapid contact with the quenching surface in order to cool the film to a stable temperature in the shortest period of time, thus minimizing crystallization of the polymer. To this end, various expedients have been employed to force the extruded web into intimate contact with the drum. Devices commonly used include jets of air applied at the edges of the extruded film or across the full width of the film by means of an air knife, and apparatus for applying an electrostatic charge to the film to cause it to more closely adhere to the quenching surface.
In spite of many varied techniques previously used to urge the freshly extruded web into contact with the quenching surface, difficulties remain which result in gauge variation in the finished film product and limit the speed at which film can be cast while maintaining excellent clarity. While electrostatic pinning is most effective for this purpose, increased speed often results in the inclusion of air bubbles between the freshly extruded film and the quenching surface, causing surface imperfections in the finished film product. In addition, perturbations in roll rotation, variations in polymer composition and temperature as well as air currents encountered immediately outside of the extrusion die have caused variation in the free melt path position and in the point at which the freshly extruded web touches down onto the quenching surface across the width of the extruded web.